This device includes an electromagnetic radiation source, such as an optical source. For example, this source may be an infrared lamp emitting a radiation with a wavelength .lambda.0.
The device also includes a cell containing a gas volume with a given concentration. The radiation derived from the lamp traverses the cell and is selectively absorbed by the gas. At the cell outlet, the radiation has a spectral density modified by the presence of absorption peaks due to the lines of the gas.
A wavelength-tunable filter is adjusted to the wavelength .lambda.0 and is temporally modified so that its spectral transmission varies according to the time and wavelength. This modulated filter receives the radiation which has traversed the gas volume and in reply provides a temporary modulation of the energy of this radiation.
A detector, such as a bolometer, receives the energy transmitted by the filter, integrates as regards the entire spectral width of the received energy source and furnishes an average response which no longer depends on the wavelength .lambda. of the radiation, but more particularly on the concentration of the gas, this response being expressed as follows: EQU E(c,t)=[Sac(c,t)+Sdc(c)]P+Oac(t)+Odc.
The values of Sac(c,t) and Sdc respectively correspond to the temporal modulation and the mean value of the integral as regards the entire spectral region of the source from the energy balance between the source and the detector through the wavelength-tunable filter and the gas having a spectral absorption band concerning the region in question.
The term P is an unknown variable coefficient of optical losses which may take account of the dust covering the device in question.
The term Oac(t)+Odc is a zero shift, mainly due to the detector, and appears in the form of two terms respectively corresponding to a time-variable portion, Oac(t), and a non-time variable portion Odc.
Now, with this type of device, the influence of the optical losses coefficient P and the drifts of the detector make it impossible to reliably determine the gas concentration.
As a consequence, it would be very interesting to be able to make up for the optical losses coefficient P and the drifts of the detector with a view to obtaining the gas concentration more reliably.